Key Stage 3 science · Chemistry – Pure and impure substances
KS3 separating mixtures and pure substances: a guide for parents
In Years 7 to 9 children learn that scientists use the word “pure” in a stricter way than the supermarket does. They find out what a mixture is, what happens when something dissolves, how diffusion is explained by moving particles, and how to separate mixtures using filtration, evaporation, distillation and chromatography. They also learn how to tell whether a substance is pure.
Pure substances, mixtures and separation, explained
In everyday language “pure” means natural or untouched, as on a carton of juice. In science it means something stricter: a pure substance contains only one kind of particle. It might be a single element, such as copper, or a single compound, such as water. Almost everything around us is a mixture of two or more substances: air, sea water, milk, soil, ink and the water from your kitchen tap. Pupils learn to sort materials into pure and impure, and to say how they know.
The substances in a mixture are not chemically joined, so each keeps its own properties and they can be separated by physical methods that involve no chemical reaction. Dissolving is an important kind of mixing. When salt goes into water, the salt is the solute, the water is the solvent, and together they make a solution. The salt has not vanished or melted: its particles have spread out between the water particles and are too small to see. A solid that does not dissolve, such as sand, is insoluble. Nothing is lost when something dissolves, so a salt solution has exactly the mass of the salt and water that went into it.
Pupils use the particle model to explain diffusion. All matter is made of tiny particles that are always moving. In gases and liquids the particles move about randomly, and over time this spreads them out from where there are lots of them to where there are few. That is why the smell of toast reaches the next room, and why a drop of food colouring spreads through still water without anyone stirring it. Diffusion is quicker in gases than in liquids, and quicker when the substance is warmer, because the particles move faster. In solids it is far too slow to notice.
The method used to separate a mixture depends on what is in it. Filtration separates an insoluble solid from a liquid: the holes in the filter paper are big enough for the liquid and dissolved particles to pass through, but not the solid pieces. Evaporation gets a dissolved solid back from a solution, because the solvent escapes as a vapour and the solid is left behind. Distillation collects the liquid itself: the solution is heated, the vapour is cooled in a condenser and runs into a clean container. It can also be used on a mixture of liquids that boil at different temperatures, such as ethanol and water, although simple school equipment only partly separates them. Chromatography separates dissolved substances, such as the dyes in a felt-tip ink, because they travel up absorbent paper at different speeds.
Pupils also learn how to identify a pure substance. A pure substance melts and boils at fixed temperatures: pure water melts at 0 °C and boils at 100 °C at normal air pressure. A mixture usually melts over a range of temperatures and boils at a different temperature from its main ingredient. Salty water, for example, boils above 100 °C and freezes below 0 °C. In chromatography a pure substance gives a single spot or line, while a mixture gives several. Children compare measurements with data tables, which is a skill they will use at GCSE.
Words your child will hear
- Pure substance
- A material made of only one kind of particle, such as a single element or compound.
- Mixture
- Two or more substances together that are not chemically joined and can be separated by physical methods.
- Solution
- A solute dissolved in a solvent. Soluble means it will dissolve; insoluble means it will not.
- Residue and filtrate
- In filtration, the residue is the solid left on the filter paper and the filtrate is the liquid that passes through.
- Diffusion
- The spreading out of particles from where there are many to where there are few, caused by their random movement.
- Distillation
- Heating a liquid until it evaporates, then cooling the vapour so that it condenses and can be collected.
- Chromatography
- A way to separate dissolved substances because they move through paper at different speeds.
What the National Curriculum says
The programme of study for Key Stage 3 says:
- Pupils should be taught about the concept of a pure substance
- Pupils should be taught about mixtures, including dissolving
- Pupils should be taught about diffusion in terms of the particle model
- Pupils should be taught about simple techniques for separating mixtures: filtration, evaporation, distillation and chromatography
- Pupils should be taught about the identification of pure substances
Objectives from the National Curriculum in England, © Crown copyright, used under the Open Government Licence v3.0.
A worked example
A dry mixture contains sand and salt. Plan how to get both back as separate solids.
- Add the mixture to water and stir. Salt is soluble, so it dissolves and its particles spread through the water. Sand is insoluble, so the grains stay as a solid.
- Filter the mixture through filter paper in a funnel. The sand is too big to pass through the paper, so it stays behind as the residue. The salt solution passes through as the filtrate.
- Rinse the residue with a little clean water to wash off any salt solution, then leave it somewhere warm to dry. This gives dry sand.
- Heat the filtrate gently in an evaporating basin until about half of the water has gone, then leave it to cool and evaporate slowly. The water escapes as vapour and salt crystals are left. In school this is done with eye protection and under the teacher’s supervision, and heating stops before the basin is completely dry so that hot salt does not spit.
- If the water were wanted as well, distillation would be used instead of evaporation. The vapour would be cooled in a condenser, which collects pure water.
Dissolve the salt in water, filter to collect the sand as the residue, then evaporate the water from the filtrate to get the salt. Distillation would also recover the water.
Common mistakes and how to help
Thinking “pure” means natural or with nothing added
Labels on food and drink use “pure” to mean unprocessed. Spring water or orange juice is called pure even though it contains many dissolved substances.
What helps: Ask “is it only one substance?” Air is natural but a mixture of gases, and distilled water is made in a factory but pure. Looking at a few labels together makes the difference clear.
Believing dissolved sugar or salt has disappeared or melted
The solid cannot be seen any more, so it seems to have gone. Children also hear “melts” used loosely for anything that goes away in a liquid.
What helps: Weigh a cup of water and a spoonful of salt before and after stirring: the total is the same. Then leave the solution on a saucer until the water has evaporated and the salt crystals reappear.
Thinking filter paper can separate salt from salty water
Filtering works well for sand in water, so children assume it works for anything mixed with a liquid.
What helps: Remind them that dissolved particles are far smaller than the holes in the paper and pass straight through. Filtered salty water is still salty, so a different method (evaporation or distillation) is needed.
Mixing up evaporation and distillation
Both start by heating a solution until the liquid turns to vapour, so they sound the same.
What helps: Ask what you want to keep. With evaporation the liquid escapes into the air and you keep the solid. With distillation the vapour is cooled and collected, so you keep the liquid.
How to help at home
- Chromatography with a felt-tip pen. Draw a dot of ink about two centimetres from the bottom of a strip of coffee filter or kitchen paper and stand the end in a shallow glass of water without letting the dot touch it. Ask your child to predict the colours, then compare black, brown and green pens, which are usually mixtures of dyes.
- Watch diffusion happen. Put one drop of food colouring into a glass of still cold water and a similar drop into a glass of lukewarm tap water. Which spreads faster, and why? For a gas example, peel an orange and ask someone across the room to say when they first smell it.
- Get the salt back. Dissolve a spoonful of salt in a little warm water, pour it on a dark saucer and leave it on a sunny windowsill for a few days. Your child can describe where the water went and what stayed behind.
- Filtering in the kitchen. Stir sand or soil into a jug of water, then pour it through a tea strainer, a coffee filter and kitchen paper in turn. Which gives the clearest water? Ask why the water still looks slightly cloudy and whether it would be safe to drink (it would not).
- Check a label. Look at bottles of mineral water, juice and milk and list what each says it contains. Ask your child to decide, with a reason, whether each is a pure substance or a mixture.
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